Salts of N-formyl-6-chlorotryptophan and α-methyl-p-nitrobenzylamine

ABSTRACT

Racemic organic carboxylic acids are efficiently resolved into their enantiomers with antipodes of α-methyl-p-nitrobenzylamine.

This is a division of application Ser. No. 404951 filed Oct. 10, 1973,now U.S. Pat. No. 3,901,915.

BACKGROUND OF THE INVENTION

It is well known that many natural products contain one or moreasymmetric centers and thus can occur in optically active forms. As arule, one optical form predominates in nature, and only this opticalisomer is responsible for the particular properties of the compound suchas taste, odor, toxicity and pharmacological properties. This generallyapplies as well to synthetic compounds, particularly in the medicinal orpharmaceutical field. That is, one optical antipode usually exhibits thedesired biological activity, whereas the other does not or does to amuch smaller extent or, in fact, may even exhibit undesirableproperties.

It has become accepted procedure in the synthesis of biologically activemolecules to prepare the desired molecule in optically active form.While, in some cases, this may be accomplished utilizing enzymatic orfermentation techniques, a chemical optical resolution is the onlymethod which can usually be used on a technical scale. For convenience,resolution is usually performed at a stage in which the intermediate orfinal product has a functional group which is suited for the resolutionprocess, usually an amine or carboxylic acid group. The racemic compoundto be resolved is reacted with an optically pure compound having acomplementary functional group to form mixtures of diastereomericcompounds, usually salts, which may then be separated due to theirdifferences in physical properties. Usually fractional crystallizationis employed. Compounds having a carboxylic acid group are commonlyresolved by reaction with an optically active amine, and compoundspossessing an amine group may be resolved by reaction with an opticallyactive acid, for example, an optically active carboxylic or sulfonicacid.

In the past, the resolution of racemates having carboxylic acid groupswas accomplished utilizing optically active amines derived from naturalsources, most notably the alkaloids such as brucine, strychnine,cinchonidine, and so forth. The use of these compounds presents manydisadvantages such as toxicity, questionable optical purity,availability (inasmuch as they must be obtained from natural sources),and the fact that they normally occur in nature in only one optical formso that only one antipode is available for a potential resolutionproblem.

In recent years, the use of simpler synthetic organic amine resolvingagents has become wide-spread. In particular, amines that have been usedextensively for optical resolution are antipodes of α-methylbenzylamine(α-phenylethylamine) and simple derivatives thereof, such as N- andN,N-lower alkyl derivatives; as well as antipodes ofα-naphthylethylamine. While these amines have been used extensively,there are many instances in which resolution does not occur eitherthrough the lack of formation of crystalline salts or the fact that thediastereomeric salts formed, even if crystalline, often do not differsufficiently in physical properties to allow facile separation.

Hence, it would be desirable to have available optically active amineresolving agents which are easily prepared, are simple to use, providehighly crystalline salts, and which will resolve a wide range of racemicacids of varying structure and type.

Both optical antipodes of α -methyl-p-nitrobenzylamine have beenpreviously described in the literature. See for example, Cope, et al.,Journal of the American Chemical Society, Vol. 92, page 1243 (1970). Theantipodes of this compound have been used, as complexes with platinum,for the partial resolution of a cylic allene, notably1,2-cyclononadiene. However, there was no suggestion to use suchcompounds for the resolution of organic carboxylic acids.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method of resolving racemic organiccarboxylic acids into their respective enantiomers by the use of anoptical antipode of α -methyl-p-nitrobenzylamine. The two antipodes of α-methyl-p-nitrobenzylamine are hereinafter referred to as the R-(+) andthe S-(-) antipodes. These materials are prepared in a number offashions as described in the literature, most commonly by nitration ofthe corresponding antipode of α-methylbenzylamine, usually afterprotection of the amino group.

The type of acids which may be resolved by use of one or the other ofthe aforementioned antipodes of α -methyl-p-nitrobenzylamine are racemicorganic carboxylic acids. These acids may contain more than one carboxylgroup. There may also be present in the molecule being resolved anynumber of functional groups such as epoxide moieties, lactones, doubleor triple bonds, aromatic rings, including heterocyclic aromaticsystems, carbonyl groups, hydroxy groups, amide groups, halogen groups,alkoxy groups, and so forth.

Acids which are particularly preferred for use in the present resolutionprocess are racemic threo-epoxyaconitic acid (I), racemicthreo-hydroxycitric acid, γ-lactone (racemic Garcinia acid lactone)(II), and racemic N-lower alkanoyl-or N-benzoyl-6-chlorotryptophans,preferably racemic N-formyl-6-chlorotryptophan (III). The structures forthese acids are presented below. ##SPC1##

The (+)-antipode of I is useful for the control of lipogenesis and alsoserves as an intermediate for the preparation of the γ-lactone of(-)-hydroxycitric acid (II) and (-)-hydroxycitric acid itself, both ofwhich are useful lipogenic control agents. Reference to the utility ofthese compounds may be found in U.S. pat. application Ser. No. 204,334,Guthrie, et al., now U.S. Pat. No. 3,810,931 and U.S. Pat. No.3,764,692, respectively. N-formyl-6-chlorotryptophan (III) can beconverted to 6-chlorotryptophan, the D-antipode of which is useful as anon-nutritive sweetening agent. See, for example, GermanOffenlegungsschrift 1,917,844 (C.A. 72: 30438c) and South African Patent69/02,303 (C.A. 75: 62341u).

Optical resolution using the present resolving agents is accomplished inthe standard manner for optical resolutions. Generally, this involvescontacting the racemic acid to be resolved with one of the antipodes ofα -methyl-p-nitrobenzylamine in an inert solvent medium. The amount ofresolving agent which may be utilized can vary greatly depending uponthe particular acid being resolved, the number of carboxyl groups permolecule of acid being resolved, and so forth. Generally, one willutilize between about 0.4 and about 10 moles of resolving agent for eachmole of racemic acid being resolved. A particularly preferred quantityof resolving agent is in the range of from about 0.5 to about 2.0 molesof resolving agent for each mole of racemic acid.

Inert solvent media that may be utilized for the present resolutionprocess include lower alkanols such as methanol, ethanol, 1-propanol,2-propanol, 1-butanol, and so forth; lower alkyl ketones such asacetone, methyl ethyl ketone, diethyl ketone, and so forth; ethers suchas diethyl ether, tetrahydrofuran, isopropyl ether, dioxane and soforth; aliphatic hydrocarbons, such as n-pentane, n-hexane, cyclohexane,and so forth; aromatic hydrocarbons such as benzene, toluene, xylene,and so forth; esters such as ethyl acetate, propyl acetate, ethylpropionate, and so forth; nitriles such as acetonitrile; mixtures of theabove; and mixtures of the above mentioned water-miscible solvents withwater. Particularly preferred solvents for the present resolution aslower alkanols such as methanol and ethanol, lower alkyl ketones such asacetone, and mixtures of these with water.

Upon treatment of the racemic acid with the optically active amine,there is formed a mixture of diastereomeric salts. These salts havedifferent physical properties and may be separated by various physicalmeans well known to one skilled in the resolution art. Such meansinclude, for example, fractional crystallization, selective extraction,and so forth. A particularly preferred method for separation of thediastereomeric salts is fractional crystallization. When employingfractional crystallization as a separation method, one will adjust thesolvent and concentration such that, upon formation of thediastereomeric salt, some of the salt will precipitate from the solventmedia, leaving the remainder in solution. Generally, the precipitatedmaterial is enriched in one of the diastereomeric salts and, of course,the filtrate or mother liquor is similarly enriched in the other isomer.Upon further crystallization of the precipitated salt, if necessary,this salt is brought to purity, that is, it will be the salt of only oneof the antipodes of the organic acid. Purity of the salt is generallyrecognized when its properties, such as melting point and opticalrotation, do not change upon further crystallization. Alternatively, ifa sample of the pure optically active acid is available, it may beconverted to optically pure salt for comparison purposes.

The optically pure salt may then be decomposed to afford the purecarboxylic acid or, alternatively, the salt may be used directly forsubsequent chemical conversions. In general, decomposition of the saltis effected either by treatment with a strong acid or with a strongbase, that is, an acid stronger than that which is being resolved, or abase stronger than the α -methyl-p-nitrobenzylamine. Displacement withan acid generally employs a mineral acid, such as hydrochloric acid orsulfuric acid. In such a case, there would be obtained the free resolvedcarboxylic acid as well as the salt of the α -methyl-p-nitrobenzylaminewith the particular acid used for the displacement. If a strong base,such as, for example, an alkali metal hydroxide, e.g., sodium hydroxide,is used for the displacement, there would be obtained the salt of theresolved carboxylic acid, for example, the sodium salt, as well as thefree α -methyl-p-nitrobenzylamine. In either case, after decompositionof the salt, the materials are easily separated due to their differencesin physical properties, particularly solubility in organic or aqueousmedia.

A particularly preferred method for decomposition of salts of carboxylicacids with α -methyl-p-nitrobenzylamine involves treatment with amineral acid such as hydrochloric acid.

The mineral acid salt of the optically active α-methyl-p-nitrobenzylamine, upon further treatment with base, affordsthe free amine which may then be extracted into organic media allowing ahigh recovery of the resolving agent without loss of optical purity.Thus, recycling of the resolving agent is accomplished in an efficientmanner making the use of such resolving agents highly economical.

Alternatively, the pure diastereomeric salt may be directly converted byother chemical reactions to different compounds. For example, the α-methyl-p-nitrobenzylamine salt of (+ )-threo-epoxyaconitic acid may bedirectly converted to the γ-lactone of (-)-hydroxycitric acid by openingof the epoxide and lactonization.

The particular salts which are formed during the resolution process willvary depending upon the carboxylic acid employed. Thus, if one utilizesa carboxylic acid having two or more carboxylic acid groups, there maybe obtained a mono-, di-, tri-, etc. salt of the carboxylic acid withthe amine. Of course, the particular salt which may be formed will alsodepend upon the quantity of resolving agent utilized.

For example, in the resolution of racemic threo-epoxyaconitic acidutilizing approximately 1.8 moles of resolving agent for each mole ofcarboxylic acid, there is obtained a high yield of a salt containing twomoles of amine for each mole of carboxylic acid, that is, two of thethree available carboxyl groups are involved in salt formation. In theresolution of racemic threo-hydroxycitric acid, γ-lactone withapproximately 1.25 moles of resolving agents for each mole of acid,there is obtained a salt which contains two moles of amine for each moleof acid.

It is noteworthy that the use of α -methyl-p-nitrobenzylamine antipodesas resolving agents leads to the formation of very highly crystallinesalts which are easily separated by fractional crystallization and thusallows a highly efficient optical resolution of carboxylic acids. Infact, in many cases, the use of α -methyl-p-nitrobenzylamine leads tothe formation of highly crystalline salts, whereas the use of the parentcompound, i.e., α-methyl-benzylamine itself, does not lead to theformation of any crystalline salt, thus being impractical for opticalresolution. This surprising and unpredictable phenomenon is of extremeimportance for the resolution of racemic compounds. Thus, for example,for the resolution of threo-epoxyaconitic acid, threo-hydroxycitricacid, γ-lactone and N-formyl-6chlorotryptophan, no crystalline saltcould be obtained utilizing antipodes of α-methylbenzylamine aspotential resolving agents, whereas high yields of highly crystallinesalts, which were easily purified to afford the desired diastereomericsalt, were obtained when an antipode of α -methyl-p-nitrobenzylamine wasemployed.

Specific examples of the use of antipodes of α-methyl-p-nitrobenzylamine for optical resolution of organic carboxylicacid are presented below. These examples are illustrative only of theinvention and are not to be construed as limitative thereof in anymanner.

EXAMPLE 1

100.0 g (0.826 mole) of R-(+) α-methylbenzylamine was added withvigorous stirring to 230 ml of acetic anhydride over a period of 1/2 hr,during which time the temperature rose to about 85°. The solution washeated to reflux for 2 hrs, cooled, and concentrated to a viscous oilunder reduced pressure. 250 ml of ice water was added to the residue andthe mixture was stirred vigorously for 1 hr. The solid product wascollected by filtration, washed thoroughly with water, and pressed asdry as possible.

The damp solid, N-acetyl derivative, was added in small portions to 650ml of 90% nitric acid at -8° to -12° over a period of 30 mins, using adry ice-acetone bath to control the temperature, which was maintained at-8° to -12° for an additional hour. The mixture was poured into 1800 mlof ice water and the pH was adjusted to 2.0 with 50% sodium hydroxidesolution (approximately 1 liter) while stirring and cooling with a dryice-acetone bath to keep the temperature below 40°. The mixture wasextracted with three 1 l. portions of methylene chloride and thecombined extracts were dried over anhydrous magnesium sulfate andconcentrated under reduced pressure to afford the nitrated N-acetylderivative (approx. 175 g).

900 ml of 20% hydrochloric acid was added to this material and thesuspension was stirred and refluxed for 9 hrs. Then the resultingsolution was cooled and concentrated under reduced pressure to dryness.To ensure dryness, the residue was treated with 100 ml of absoluteethanol and concentrated to dryness. This operation was repeated twicemore, then 300 ml of absolute ethanol was added and the mixture wasstirred for 1 hr. Filtration and drying at 60° under reduced pressuregave 83.2 g of the hydrochloride of R(+)- α-methyl-p-nitrobenzylamine,m.p. 240°-242° dec. This product was purified further by refluxing itwith 200 ml of absolute ethanol for 1 hr, cooling to 10°, and filtering,whereupon 76.4 g of purified product was obtained, m.p. 243°-245° dec.The free amine was recovered by addition of the hydrochloride to 450 mlof 1 N sodium hydroxide solution and extraction three times with 500 mlof methylene chloride. The extracts were dried over anhydrous magnesiumsulfate and the solvent was removed by distillation under reducedpressure. Distillation of the residual oil gave 58.9 g of the pureamine, b.p. 119°-120°/0.5 mm,[α]_(D) ²⁴ =+16.5° (c = 3% in ethanol). TheS(-) amine was prepared in identical fashion starting with S(- )α-methylbenzylamine.

EXAMPLE 2

38.85 g (200 mmoles) of racemic threo-epoxyaconitic acid dissolved in250 ml of methanol-water (98:2 v/v) was treated with 59.80 g (360mmoles) of R(+)- α-methyl-p-nitrobenzylamine in 150 ml of the samesolvent. The warm solution immediately began to deposit solid and wasleft at room temperature overnight. The mixture set to a solid mass,which was mashed and filtered. The collected solid was rinsed with freshsolvent and dried in a vacuum oven at 45° overnight to give 50.5 g solidwith [α]₄₃₆ ²⁵ + 13.07° (c = 2.035% in water), approx. 87.3% opticallypure. The solid was stirred and refluxed gently as a slurry with 400 mlof the same solvent for 2 hr, then allowed to cool to room temperaturewith stirring overnight to give 44.78 g of solid with [α]₄₃₆ ²⁵ +14.83°(C = 2% in water). This salt contained 2 moles of amine per mole of acidand was 99.1% optically pure by comparison with an authentic sampleprepared by reacting optically pure (+ )-threo-epoxyaconitic acid withR(+) α-methyl-p-nitrobenzylamine.

EXAMPLE 3

9.12 g (48 mmoles) racemic threo-hydroxycitric acid γ -lactone wasdissolved in 80 ml of ethanol and to this a solution of 10 g (60 mmoles)R-(+ )-α-methyl-p-nitrobenzylamine in 20 ml methanol was added. Thesolution was stirred overnight and the resulting solids were filteredand washed with methanol to give 9.3 g of crude product [α]_(D) ²⁵ =+30.7°. The crude salt was purified by refluxing in 100 ml ethanol for 3hrs, then cooling the mixture and filtering the solid to give 7.56 g ofpure salt [α]_(D) ²⁵ = +38.2°. The salt contained 2 moles of amine permole of acid.

9.7 g (18.6 mmole) of the above salt was suspended in 75 ml of diethylether and to this 50 ml of a 1 N ethereal hydrogen chloride solution wasadded. The mixture was stirred at room temperature for 20 minutes underanhydrous conditions. The solid was recovered by filtration and washedwith ether to give 7.7 g of the hydrochloride of R-(+)-α-methyl-p-nitrobenzylamine, m.p. 245°-247°. The combined filtrates wereconcentrated in vacuo to give 3.35 g of lactone. This material wascrystallized from ethyl acetate-CCl₄ to give 2.6 g of the γ-lactone of(- )-threo-hydroxycitric acid [α]_(D) ²⁵ = +106.1° (1% H₂ O): m.p.179°-180.5°. A second crop gave 0.33 g of lactone, m.p. 178°-180°,[α]_(D) ²⁵ +106.4°.

EXAMPLE 4

65.7 g (0.122 mole) of the bis R-(+)-α-methyl-p-nitrobenzylamine salt of(+ )-threo-epoxyaconitic acid (98-99% optical purity) and 73 ml of 0.5 NHCl (0.365 mole) were heated at reflux for 16 hrs. The solution wascooled and extracted with EtOAc (2 × 100 ml). the organic layers werebackwashed in turn with water (50 ml) and the combined aqueous layerswere evaporated to dryness under reduced pressure. The residue wastriturated with EtOAc (3 × 100 ml) and the solids were recovered byfiltration to give 44.8 g (91.4%) of recovered R(+)-α-methyl-p-nitrobenzylamine hydrochloride (m. p. 246°-7°). The filtratewas dried over MgSO₄, evaporated in vacuo and the residue was heated at75°-80° in vacuo for 1/2 hr. to ensure lactonization. Crystallization ofthe residue (21 g) from EtOAc-CCl₄ furnished the γ -lactone of (-)-threo-hydroxycitric acid in two crops: 6.0 g [m.p. 174°-6° ; [α]_(D)²⁵ +105.6°]and 3.2 g [168°-173°;[α]_(D) ²⁵ +100.5°]. The crops werecombined and recrystallized from EtOAc-CCl₄ (after decolorization usingacid-washed Norit Sv charcoal) to give the γ-lactone in two crops; m.p.178°-180°, [α]_(D) ²⁵ +106.2°, and m.p. 178°-180°, [α]_(D) ²⁵ +105.9°.

EXAMPLE 5 dl-N-formyl- 6-chlorotryptophan

15 g(628 mM) of dl-6-chlorotryptophan was treated with 15 ml ofacetic-formic anhydride. An exothermic reaction ensued, accompanied bysolidification of the reaction slurry. Excess solvents were distilledoff under vacuum and the resultant solids, after first leaching withdilute acid, were recrystallized from ethyl acetate to yield 12.45 g,74%, of product, m.p. 181°-3° dec.

EXAMPLE 6 N-formyl-L-6-chlorotryptophan R-(+)α-methyl-p-nitrobenzylamine salt

12.45 g (46.7 mM) of dl-N-formyl-6-chlorotryptophan and 7.76 g of R-(+)-α-methyl-p-nitrobenzylamine were dissolved in 155 ml of acetone. Thecrude solids which separated out were recrystallized three times fromethanol to yield 1.94 g, product, m.p. 133°-4°, [α]_(D) ²² = +23.0° (c321, methanol).

EXAMPLE 7 N-formyl-L-6-chlorotryptophan hydrate

1.94 g (4.5 mM) of N-formyl-L-6-chlorotryptophanR(+)-α-methyl-p-nitrobenzylamine salt was slurried with 50 ml of 0.1 NHCl. The resultant solids were filtered off and crystallized fromwater-ethanol to yield 1.09 g of product, m.p. 143°-5°, [α]_(D) ²² =+47.1° (c=1, methanol).

EXAMPLE 8 L-6-chlorotryptophan

5.16 g (18.1 mM) of N-formyl-L-6-chlorotryptophan hydrate and 100 ml of2 N acetic acid was refluxed for 22 hrs. The reaction mixture wasconcentrated in vacuo to solids which were leached with ethanol andcrystallized from water to give 2.41 g of product, m.p. dec. with gasevolution 264° C, [α]D 22 = -15.4° (c=1, glacial acetic acid), [α]_(D)²² = -28.5° (c =1, methanol).

EXAMPLE 9 N-formyl-D-6-chlorotryptophanS-(-)-α-methyl-p-nitrobenzylamine salt

The acetone filtrate from which the crude product from Example 6separated out was concentrated in vacuo to dryness. The resultant oilyresidue was suspended in water and the mixture was adjusted to pH 1 withdilute HCl and extracted with ethyl acetate. The ethyl acetate extractswere concentrated in vacuo to an oil and treated with dilute HCl to give6.94 g of partially resolved chlorotryptophan, [α]_(D) ²² = - 18.9°(c=1, methanol). This material and 4.79 g ofS(-)-α-methyl-p-nitrobenzylamine were dissolved in 85 ml of acetone. Thecrude solids which separated out were recrystallized one time frommethanol to yield 4.65 g of product, m.p. 183°-4°, [α]_(D) ²² = -27.4°(c=1, MeOH).

EXAMPLE 10 N-formyl-D-6-chlorotryptophan

4.65 g (10.1 mM) of N-formyl-D-6-chlorotryptophanS(-)-α-methyl-p-nitrobenzylamine salt was slurried with about 65 ml of0.2 NHCl. The resultant solids were filtered off and crystallized fromwater-ethanol to yield 2.32 g of product, m.p. 143°-5° C, [α]_(D) ²² =-47.0° (c=1, MeOH).

EXAMPLE 11 D-6-chlorotryptophan

1.06 g (5.82 mM) of N-formyl-D-6-chlorotryptophan and 34 ml of 2 Nacetic acid was refluxed for 22 hrs. The reaction mixture wasconcentrated in vacuo to solids which were leached with a small amountof ethanol and crystallized from water to give 0.65 g of product, m.p.dec. with gas evolution 264°, [α]_(D) ²² = +15.0° (c=1, glacial aceticacid), [α]_(D) ²² = +28.2° (c=1, MeOH).

EXAMPLE 12

Following the procedure of Example 6, but usingS(-)-α-methyl-p-nitrobenzylamine as the initial resolving agent, therewas obtained as a precipitate N-formyl-D-6-chlorotryptophanS(-)-α-methyl-p-nitrobenzylamine salt, m.p. 183°-4°, [α]_(D) ²² = -27.4°(c=1, MeOH). The salt was decomposed, according to the procedure inExamples 7 and 10 to afford N-formyl-D-6-chlorotryptophan, m.p. 143°-5°,[α]_(D) ²² = -47.0° (c=1, MeOH), which was deformylated according to theprocedures in Examples 8 and 11 to afford D-6-chlorotryptophan, m.p.dec. 264°) [α]_(D) ²² = +15.0° (c=1, glacial acetic acid), [α]_(D) ²² =+28.2° (c=1, MeOH). From the filtrate following the initialprecipitation there was obtained, after following the procedures inExamples 9-11, N-formyl-L-6-chlorotryptophanR(+)-α-methyl-p-nitrobenzylamine salt, N-formyl-6-chlorotryptophanhydrate, and N-formyl-6-chlorotryptophan, respectively.

1. The salt of N-formyl-D-6-chlorotryptophan withS(-)-α-methyl-p-nitrobenzylamine.